Non-endochondral bone formation (osteogenesis) occurs either as lamellar or woven bone. Lamellar bone forms during normal modeling and remodeling, whereas woven bone forms as part of an injury response (e.g., fracture healing, distraction osteogenesis, fatigue/stress fractures). Little is known about what leads the same cell type (the osteoblast) to directly produce lamellar bone in one case and woven bone in another. An advantage of woven bone is that it can quickly increase bone width and moment of inertia, leading to rapid restoration of whole-bone stiffness and strength. If the pathway(s) of non-endochondral woven bone formation could be better understood, it may lead to the discovery of novel strategies for treating disorders of low bone mass and strength. Recent work has demonstrated that a rapid woven bone response occurs after a single bout of damaging fatigue loading. In light of observations that new blood vessel formation (angiogenesis) also occurs after fatigue loading of bone, we hypothesize that the angiogenic response may influence the nature of the osteogenic response, i.e. woven versus lamellar. Our long-term goal is to determine the mechano-biological pathway that leads to rapid non-endochondral bone formation after mechanical loading. We hypothesize: (1) mechanical loading-induced woven bone formation is a response to structural damage rather than a response to cyclic deformation; and (2) the osteogenic response to damaging fatigue loading is dependent on the vigor of the angiogenic response. Using the rat ulnar loading model, in Specific Aim 1 we will: (A) determine the osteogenic response to fatigue (i.e., dynamic) loading as a function of the level of structural damage; (B) determine the osteogenic response to creep (i.e., static) loading as a function of the level of structural damage; and (C) correlate patterns of bone formation with finite element-predicted patterns of bone strain. In Specific Aim 2, we will: (A) assess changes in bone vasculature following damaging fatigue loading; (B) assess the spatial pattern of expression of important osteogenic (e.g., BMP-2) and angiogenic factors (e.g., VEGF); and (C) assess the ability of anti-angiogenic compounds to block woven bone formation after damaging fatigue loading. Taken together, these studies will establish important mechanical and angiogenic factors that mediate the distinct processes of lamellar versus woven bone formation. They will extend our understanding of the osteogenic response to skeletal stress injuries (fatigue fractures) and will serve as a basis for future development of novel strategies for rapid bone formation to augment weak or damaged bones. [unreadable] [unreadable]